258 Development of a Standardised Agar Medium 



can take place on these media, but only after much more intense drying 

 than is needed to induce it on nutrient agar. On the media containing 

 peptone, lemco or tyrosine, spores were produced in large numbers 

 within 14 days. It would appear that these substances contain a com- 

 ponent which renders spore formation more easily induced. But even 

 on these media, spores were not produced on platings until drying of the 

 surface water had stopped the rapid spread of the organisms. Experi- 

 ments were also made with various carbohydrates, which, however, 

 were without effect on spore formation. While, therefore, the formation 

 of spores can be retarded by certain conditions of nutrition, it appears 

 on agar platings as a reaction to drying of the surface moisture, and it 

 would seem that it cannot readily be induced until this immediate cause 

 begins to operate^. 



In applying our knowledge of the mode of growth of B. dendroides, 

 in an attempt to check its spreading over agar plates, the following facts 

 must be borne in mind. 



A. The duration of the period of rapid spreading is limited by the 

 surface moisture of the agar and terminates when this disappears. 

 Methods of drying the agar surface so as to curtail this period do not 

 appear practical in routine work involving a large number of platings. 

 It is not at present possible to shorten the period of spreading by hasten- 

 ing the incidence of spore formation. 



B. The rapidity of spreading during the existence of the surface 

 moisture film is influenced by two characters : 



(1) The motihty of the organism, 



(2) Its rate of multiplication. 



It has not been found possible to reduce the motility of the organism 

 on platings during this period by any change in the composition of the 

 medium^. The rate of multiplication, on the other hand, is greatly in- 

 fluenced by the food supply, and it seemed probable that by checking 

 this during the period of spreading, the area of spread could be much 

 reduced. 



It is known that the content of organic matter of the medium 

 influences the formation of spreading colonies on plates (3). I therefore 

 decided to investigate the influence of the organic nitrogen constituent 



^ Cultural conditions liable to inhibit the development of other soil organisms on the 

 plates are not here considered. 



^ H. Braun (i6) found that B. proteus, if grown on agar media in which the nutrient 

 material and salts were reduced to yu the normal concentration, lost its flagella and con- 

 sequently formed non-spreading colonies. Unfavourable food conditions, however, do not 

 appear to influence the motility of B. dendroides, though affecting its multiplication rate. 



